Epitaxial TiC (001) layers: Phase formation and physical properties vs C-to-Ti ratio

Titanium carbide films are deposited onto MgO(001) by reactive magnetron sputtering in Ar/CH ₄ mixtures at 1100 °C with a varying CH ₄ fraction f _CH4 = 0.4–8%, yielding C-to-Ti ratios x = 0.08–1.8. The microstructure is dominated by an epitaxial rock-salt structure TiC _x(001) matrix which contains secondary hcp Ti and graphitic/a-C:H for x ≤ 0.24 and x ≥ 1.5, respectively. First-principles calculations indicate negligible interstitial C in the cubic phase, a large equilibrium phase-field x = 0.5–1 for the cubic structure which is extended to x < 0.5 by entropic stabilization, and a predicted C-solubility in hcp Ti of x = 0.06 at 1100 °C. The measured relaxed lattice constant increases from a _o = 0.4304 nm for TiC _0.5 to 0.4325 nm for TiC _1.0, in excellent agreement with predictions. However, d a _o/d x for x < 0.5 and x > 1.0 is much smaller than predicted for C vacancies and interstitials in rock-salt TiC _x, respectively, confirming secondary phase formation and indicating a minimum x = 0.46 in the cubic phase. The hardness and elastic modulus of phase-pure TiC _x(001) increases from H = 24 GPa and E = 304 GPa for TiC _0.5 to H = 31 GPa and E = 462 GPa for TiC _1.0, which is attributed to an increasing bonding ionicity. H and E decrease with x < 0.5 and x > 1.0 due to secondary phase softening which is well described by an effective medium with homogenous stress. The electrical resistivities for TiC _0.5 and TiC _1.0 are 168 and 83 μΩ cm at 298 K, and 158 and 72 μΩ cm at 77 K, indicating electron scattering at random anion vacancies for x = 0.5 but also dominant defect scattering for stoichiometric TiC _1.0.